Organic light-emitting diodes have undergone a very rapid development in recent times. Even though they were developed for the first time in 1987 for vapor-deposited organic materials (cf. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1978)) for deposited polymer materials from liquid solution (cf. J. H. Burroughes et al., Nature 347, 6293 (1990)), excellent parameters for the efficiency and life service duration of organic light-emitting diodes were already achieved in recent years. In particular, efficiencies of over 80 lumen/w were successfully obtained for green-emitting light-emitting diodes (cf. He et al., Appl. Phys. Lett. 85, 3911 (2004)). Comparably good values were already achieved also for red-emitting and blue-emitting organic light-emitting diodes.
As also the lifetime of these systems has grown very quickly and, in the meantime, values of 10,000 hours for some material systems have even been significantly exceeded, organic light-emitting diodes also appear interesting for applications in lighting systems. The essential advantages of the organic light-emitting diodes, in addition to the possible high efficiency, which even today exceeds that of incandescent bulbs and will in future possibly reach the efficiency of fluorescent tubes, are the possibility of realizing a large-surface lighting unit through which a very glare-free and for many applications ideally suitable light can be generated.
The conventional structural arrangement of organic light-emitting diodes comprises a transparent substrate, glass in most cases, which is coated with a transparent anode that is frequently formed from indium tin oxide (ITO). Onto this, active organic layers are deposited and, subsequently and additionally, a metallic cathode for electrical contacting is deposited. If some volts are applied between the metallic cathode and the transparent anode, the light-emitting diode emits the light through the substrate.
There are also technical concepts that allow the construction of light-emitting diodes, which are at least partially transparent. For this purpose, for example, the cathode can also be provided with a transparent conductive metal or with a thin and partially transparent metal layer (cf. Gu et al., Appl. Phys. Lett. 68, 2606 (1996); Parthasaray et al., Appl. Phys. Lett. 76, 2128 (2000)). In the arrangements proposed up to now, the light of the light-emitting diode is then emitted in both directions, where the exact ratio of the light volumes radiated in both directions depends on the structural configuration of the layer arrangement. This is useable for some applications, for example displays, which are to be read from both sides.
However, it is frequently disadvantageous if a transparent light-emitting diode emits to both sides. On the other hand, it would be very favorable if it were possible to realize organic light-emitting diodes, which are transparent and at the same time emit with significant preference in one direction. With regard to the initial concepts known from the literature, the light intensities in both directions are approximately equal (cf. Gu et al., Appl. Phys. Lett. 68, 2606 (1996); Parthasaray et al., Appl. Phys. Lett. 76, 2128 (2000)).